Polyisocyanates containing blocked isocyanate groups and their use in polyurethane lacquers are known (cf., for example, Kunststoff-Handbuch, Vol. VII, Polyurethane, Carl Hanser Verlag, Munich (1966) pages 11-13, 21 et. seq.). For their use in polyurethane lacquers, these blocked polyisocyanates have to satisfy the following principal requirements:
(1) relatively low split-off temperatures for the shortest possible stoving time; PA1 (2) little or no yellowing during stoving and brief overstoving; PA1 (3) substantially universal compatibility with the reactants; and PA1 (4) a substantially or, better still, completely nontoxic blocking agent having the lowest possible molecular weight to prevent excessive ballast from entering the lacquer and, during stoving, the atmosphere. PA1 (a) at least one polyhydroxyl compound having a molecular weight in the range of from about 400 to 50,000, optionally in admixture with polyhydroxyl compounds having a molecular weight below about 400, and PA1 (b) a polyisocyanate component containing blocked isocyanate groups, the quantitative ratios between components (a) and (b) corresponding to an equivalent ratio of hydroxyl groups in components (a) to the blocked isocyanate groups in components (b) of from about 0.8:1 to 1.6:1, distinguished by the fact that a polyisocyanate mixture obtainable by the process according to the present invention is used as component (b). PA1 (1) 3-isocyanato methyl-3,5,5-trimethyl cyclohexyl isocyanate, also referred to hereinafter as "isophorone diisocyanate" or "IPDI"; PA1 (2) polyhydroxyl compounds having a molecular weight in the range of from about 62 to 300 and mixtures of polyhydroxyl compounds having an average molecular weight of from about 62 to 300; and PA1 (3) blocking agents containing isocyanate-reactive C-H-groups.
Unfortunately, conventional blocked polyisocyanates and donor systems only satisfy some of the above-mentioned requirements. For example, polyurethanes based on phenol-blocked aromatic polyisocyanates show a tendency towards yellowing and have poor weather resistance. Although aliphatic and cycloaliphatic polyisocyanates do not have this disadvantage, it has hitherto not been possible to find any blocked polyisocyanates having aliphatically-bound isocyanate groups which are compatible with substantially all the polyhydroxyl compounds commonly encountered in polyurethane chemistry. The blocked polyisocyanates according to DT-OS No. 2,342,603 are also unsatisfactory with regard to their compatibility with the polyhydroxyl compounds (cf. Comparison Examples 1 and 8). In addition, one disadvantage of the blocked polyisocyanates according to DT-OS No. 2,342,603 which should not be underestimated is the fact that their production involves the use of a monomer-free biuret polyisocyanate which in turn may only be obtained by a relatively complicated two-stage operation (preparing a mixture of biuret polyisocyanate and monomeric diisocyanate and subsequently removing the monomeric diisocyanate.)
The present invention provides a much simpler method, by comparison with the process according to DT-OS No. 2,342,603, of obtaining blocked polyisocyanates having aliphatically and cycloaliphatically bound (blocked) isocyanate groups which optimally satisfy all the practical requirements (1) to (4) mentioned above.